A. Technical Field
The present invention relates to a catalytic gas phase oxidation reaction. Specifically, the present invention relates to a catalytic gas phase oxidation reaction with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor packed with catalysts.
B. Background Art
In cases where, in the catalytic gas phase oxidation reactions with the molecular oxygen or the molecular-oxygen-containing gas by using the fixed-bed multitubular reactor packed with catalysts, (A) at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether is used as a raw material to produce an unsaturated aldehyde corresponding to the raw material, (B) an unsaturated aldehyde is used as a raw material to produce an unsaturated carboxylic acid corresponding to the raw material, and (C) at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether is used as a raw material to produce an unsaturated carboxylic acid corresponding to the raw material, then these catalytic gas phase oxidation reactions are accompanied with extremely exothermic reactions, so a local portion having an extraordinarily high temperature (which may hereinafter be referred to as “hot spot portion”) occurs in a catalyst layer.
When the hot spot portion has a high temperature, the catalyst excessively causes the oxidation reaction at this hot spot portion, thus lowering the yield of the objective product, and, in the worst case, the catalyst causes a runaway reaction. A catalyst as located at the hot spot portion is exposed to the high temperature, and therefore there is accelerated the deterioration of the catalyst, such as changes of physical properties and chemical properties of the catalyst to result in lowering its activity and the selectivity of the objective product. Particularly, in the case of a molybdenum-containing catalyst (e.g. a molybdenum-bismuth-iron-containing catalyst, a molybdenum-vanadium-containing catalyst; hereinafter the same), the composition and properties of the catalyst tend to change due to sublimation of the molybdenum component, and therefore the deterioration extent of the catalyst is large.
The above problems are more striking in the case of carrying out the reaction with the gas pressure (hereinafter, the gas pressure refers to a “gas pressure at the gas outlet of each reaction tube in the fixed-bed multitubular reactor”.) made high and in the case of carrying out the reaction with the space velocity made large, or with the concentration of the raw material gas made high, for the purpose of enhancing the productivity of the objective product.
The above problems are explained again below. If attention is directed to the entirety of the catalyst layer as packed in the reaction tube, then the catalyst as located at the hot spot portion causes the oxidation reaction excessively, and besides, this catalyst is more rapidly deteriorated than catalysts as located at the other portions. Particularly in longtime use, the yield of the objective product is greatly lowered, so its production can be difficult to stably carry out.
In order to cope with such problems, there is proposed a process in which it is arranged that the size (occupation volume) of a catalyst packed in a reaction tube should become smaller in order from the side of the inlet of such as raw material gas toward the outlet side (e.g. refer to patent documents 1 and 2 below). There are also its examples industrially carried out.
In addition, there are proposed: a process which involves lowering the supporting ratio of an active component of a catalyst packed on the side of the inlet of such as raw material gas (e.g. refer to patent document 3 below); and a process which involves packing a catalyst of which the activity has been lowered by adding an alkaline metal (e.g. refer to patent document 4 below).                [Patent Document 1] JP-B-084400/1995 (Kokoku)        [Patent Document 2] JP-A-241209/1997 (Kokai)        [Patent Document 3] JP-A-010802/1995 (Kokai)        [Patent Document 4] JP-A-336060/2000 (Kokai).        
However, in view of high levels (e.g. high gas pressure conditions) required by recent years' technological progress in point of enhancing the productivity of the objective product, even such prior processes still would not be said to be sufficient to suppress the thermal accumulation (rise of temperature) at the hot spot portion. In addition, even if the use of a catalyst of a larger size (occupation volume) is necessary for making the catalytic activity in a predetermined reaction zone come in a lower range, there is a limitation on the size (length) of the tube diameter of each reaction tube. Therefore, for example, if an attempt is made to pack a catalyst of which the maximum particle diameter is slightly smaller than the tube diameter, then there is a case where the reaction tube could be unfavorably bridged (clogged up) with this catalyst anywhere inside the reaction tube. In such a case, for example, it is difficult to even out the packing amounts of the catalyst in all reaction tubes, so that the yield of the objective product is greatly decreased, or that its quality is widely dispersed.